1. Field of the Invention
The invention concerns a method of and an arrangement for generating pulses of a predetermined time relation during given pulse intervals with a very high time resolution.
2. Description of the Prior Art
For testing memory products, a suitable pulse pattern is applied and the behavior of the memory product in response to such pulse patterns is recorded and compared with theoretical desired values. This comparison provides data on whether the memory product operates as required or on whether there are any defects.
The pulse train to be generated is initially predetermined in theory by the specifications for the memory product and is generated using a pulse generator.
In detail it is pointed out that optimum testing should take into account that an event, attributable to the respective product to be tested, may make it necessary to replace one pulse train by another. This means, upon the occurrence of such an event, the original pulse train has to be replaced by a new one. Switching from one pulse train to another requires time. For technical reasons prior art systems could not be switched without delays, and a new pulse train could be introduced only after transients had decayed during which time the conditions in the product to be tested might have changed.
To eliminate this waiting period U.S. Pat. No. 4,203,543 (German Offenlegungsschrift No. 27 46 743) provided a method of and an arrangement for generating immediately successive pulse trains, characterized in that before a particular count is reached, the down counter is loaded with a new initial count from memory, loading being effected at a time at which the down counter would have reached a zero count.
A 100 MHz oscillator with a 10-nanosecond (ns) time raster driving the arrangement for implementing this method, permitted the generation of pulses of a predetermined time relation (with a 1 ns time raster) during given pulse intervals with a 10 ns time raster. However, this 10 ns time raster was insufficient for testing fast memory products.
To test such fast memory products, U.S. Pat. No. 4,263,669 (German Offenlegungsschrift No. 28 29 709) provided a method of generating immediately successive pulse cycles, wherein before a particular count is reached, the down counter is loaded with a new initial count from memory, loading being effected at a time at which the down counter would have reached a zero count. This method is characterized in that the start of a count step of the down counter is delayed by integral multiples of the count clock, and that a pulse is subjected to a delay with a high time resolution to provide a cycle start pulse.
The arrangement described in this U.S. Pat. No. 4,263,669 also permits pulse intervals in the 1 ns time raster. However, as a result of the circuits used, the start of a new pulse interval invariably had to coincide with the start of a 10 ns time raster derived from a quartz oscillator. Therefore, idle time of about 20 ns, which occurred either before the start of a pulse to be newly generated or after the decay of a pulse previously generated up to the start of the subsequent pulse interval, had to be tolerated.
The reason for this was that the 1 ns delay values for the pulses to be generated had to be loaded into the respective counters only at times at which no pulses were generated or at which a pulse previously generated had decayed.
Thus, it was not possible to successively generate pulse intervals with a 1 ns time resolution.
U.S. Pat. No. 4,389,614 (German Offenlegungsschrift No. 30 23 699) avoided such idle times by permitting the successive generation of pulse intervals and pulses having a high time resolution. Using known circuit technologies and the circuit speeds they afford, this patent taught that pulse intervals and pulses could be generated with a time resolution of 1 ns. In this prior art, the signals designating the start and the end of a pulse interval were generated by an oscillator for coarse raster time values and a delay circuit with selectively addressable delay taps. The signals designating the pulse intervals were alternately applied to one of two paths such that the signal designating the start of the respective pulse interval coincided with the coarse time raster provided by the oscillator. For each path, the leading and the trailing edge of a pulse to be generated during a pulse interval were derived from oscillator clock driven counters which are loaded with a new count from memory when a particular count is reached. The counters for generating the leading and trailing edges were connected to one common delay circuit each with memory-controlled, selectively addressable delay taps and delay taps for a fine time raster, respectively. The pulse data generated on both paths were merged on a common line.
The circuit according to U.S. Pat. No. 4,389,614 thus permitted the generation of pulse intervals with a resolution of 1 ns.
However, switching between taps of the delay line involved waiting periods which prevented the generation of immediately successive pulse intervals. These waiting periods can be large for two reasons; first, when switching from one delay line tap to another it may happen, for example, that a dropping voltage is stopped from dropping and is initially forced to rise. In such a case, there is an additional waiting time until a new edge suitable for counting is available at the new delay line tap; and second, when known fine delay lines with taps staggered by 100 picoseconds (ps) are used an additional waiting period will be incurred by switching the 10 ns base frequency. As these fine delay lines consist of LC devices, which when series-connected do not reach the frequency and the characteristic impedance stability of a coaxial cable in the LC equivalent circuit diagram, additional problems are encountered with transients.
The above-described method according to U.S. Pat. No. 4,389,614 thus requires elaborate circuit means necessary for the 2-path method. Also adjustment tolerances for the taps of the delay lines propagate through the entire system for generating the desired pulse trains, causing inaccuracies, i.e. variations of the reference points designating the start of the pulse intervals, and pulses, whose length is less than two successive pulse intervals minus waiting periods cannot be generated.
In summary it may be said that this 2-path method does not permit the generation of pulse intervals and pulses with a higher time resolution than 1 ns, even if it were possible to replace the delay lines by lines with a 100 ps time stagger. Generally, such 100 ps staggered delay line taps would be excessive and inaccuracies would propagate through the entire system, jeopardizing the time requirements for the pulse trains to be generated.